To hear or not to hear: How Selective Tidal Stream Transport Interferes with the Detectability of Migrating Silver Eels in a Tidal River

Acoustic telemetry provides valuable insights into behavioural patterns of aquatic animals such as downstream migrating European eels ( Anguilla anguilla ). The behaviour of silver eels during the migration is known to be influenced by environmental factors, yet so is the performance of acoustic telemetry networks. This study focusses on quantifying the impact of these environmental factors on both, migration behaviour and receiver performance, in order to determine possible limiting conditions for detecting tagged eels in tidal riverine areas and estuaries. A dominance analysis of the selected models describing migration speed, activity and receiver performance was conducted following 234 silver eels that were tagged with acoustic transmitters and observed by a receiver network in the Ems River during two subsequent migration seasons. The results suggest a passive locomotion of silver eels during their downstream migration by taking advantage of selective tidal stream transport (STST) It is further shown that water temperature, salinity, turbidity, precipitation, and especially current velocity were major parameters influencing migration activity and speed. At the same time, analyses of the detection probability of tagged eels under varying environmental conditions indicated a decreased receiver performance during high current velocities, resulting in a coincidence of high migration activity and reduced detection probability. Correspondingly, there is a risk that particularly during phases of increased activity, due to limited telemetry performance, not all fish will be detected, resulting in an underestimation of migration activity. To avoid misleading interpretations and underestimates of migration numbers of eels and other migratory fish using STST, this study highlights the need to conduct range tests and adjust the receiver placement in areas and conditions of high current velocities.